<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script src="https://unpkg.com/vue@next"></script>
</head>
<body>
  <div id="app">
    <custom-slot>
      <div>add another thing</div>
    </custom-slot>
  </div>

  <script>
    const app = Vue.createApp({})

    app.component('custom-slot', {
      template: `
      <div>
        <p>Something!</p>
        <slot></slot>  
      </div>
      `
    })

    app.mount('#app')
  </script>
</body>
</html>